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In IH with and without long sleep time, sleep drunkenness and few or not awakenings during sleep are typical characteristics. The difference between the condition with long sleep time and without is the sleep time. If it is prolonged, which means more than 10 hours, the condition is called with long sleep time but if the sleep time is not prolonged, which means more than 6 hours but less than 10 hours, then the condition is called without sleep time (Table 4).

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Table 4. Diagnostic criteria of idiopathic hypersomnia (IH) according to the International Criteria of Sleep Disorders-second version (ICSD-2) (American Sleep Disorders Association, 2005).

ICSD-2 diagnostic criteria of idiopathic hypersomnia (IH).

I. Criteria for IH with long sleep time.

A. The patient has a complaint of excessive sleepiness occurring almost daily for at least three months

B. The patient has prolonged nocturnal sleep time (more than 10h) documented by interview, actigraphy or sleep logs. Waking up in the morning or at the end of naps is usually laborious

C. Nocturnal polysomnography has excluded other causes of daytime sleepiness D. The polysomnogram demonstrates short sleep latency and a major sleep period

that is prolonged to more than 10h in duration

E. If an MSLT is performed following overnight polysomnography, a mean sleep latency of less than 8 min is found and fewer than two SOREMP are recorded Mean sleep latency in IH with long sleep time has been shown to be 6.2±3 min F. The hypersomnia is not better explained by another sleep disorder, medical or

neurological disorder, mental disorder, medication use or substance use disorder

II. Criteria for IH without long sleep time

A. The patient has a complaint of excessive sleepiness occurring almost daily for at least three months

B. The patient has normal nocturnal sleep (greater than 6h but less than 10h) documented by interviews, actigraphy or sleep logs

C. Nocturnal polysomnography has excluded other causes of daytime sleepiness

D. The polysomnogram demonstrates a major sleep period that is normal in duration (greater than 6h but less than 10h)

E. An MSLT following overnight polysomnography demonstrates a mean sleep latency of less than 8 min and fewer than two SOREMP. Mean sleep latency in IH has been shown to be 6.2±3 min

F. The hypersomnia is not better explained by another sleep disorder, medical or neurological disorder, mental disorder, and medication use or substance use disorder

2.3.3. Clinical symptoms

This sleep disorder presents the characteristics of severe excessive daytime sleepiness with not-refreshing naps of up to three or four hours, a major sleep episode prolonged to at least 10 hours typically even 12 to 14 hours with few or no awakenings. The patients have problems waking up in the morning or at the end of a nap with sleep drunkenness upon awakening (Billiard, 2009; American Academy of Sleep Medicine, 2005). Although these symptoms are features of IH and narcolepsy, they are also present in other illness such as psychiatric disorders and hypersomnia linked to other neurological disorders (Kryger et al., 2005). Some IH patients have spontaneous improvement for more than 6 months.

However this cannot be predicted because the clinical and polysomnographic features are similar between individuals who improve and those who do not (Anderson, Pilsworth, Sharples, Smith & Shneerson, 2007).

EXCESSIVE DAYTIME SLEEPINESS, NAPS AND SLEEP EFFICIENCY

Unlike in narcoleptic patients, EDS in idiopathic hypersomniacs is not imperative yet permanent (American Academy of Sleep Medicine, 2005). The consequence is frequent naps that are not refreshing. Some patients report that they avoid the naps as much as possible because of the subsequent sleep drunkenness; nonetheless, this can increase automatic behaviours (Kryger et al., 2005). Although planned naps are unfavourable, authors mention that some IH patients can have short and refreshing naps (Bassetti et al., 1997). The sleep of IH patients is typically undisturbed. In comparison with narcoleptics, the sleep efficiency of IH is significantly better (Anderson et al., 2007), with fewer awakenings and reduced wake duration (Bruck & Parkes, 1996).

SLEEP DRUNKENNESS

Sleep drunkenness is a typical symptom of IH and is characterized by a difficulty to wake up. It is associated with automatic behavior, confusion and recurrent returns to sleep (Anderson et al., 2007). Patients commonly report that they do not awake with clocks (American Academy of Sleep Medicine, 2005) which render it necessary to use other creative ways to get out of bed. The sleep inertia also called the “time to get going” can be considerably long during the morning (Kryger et al., 2005).

OTHER ASSOCIATED SYMPTOMS

IH patients often have SP and HH although these symptoms are commonly described in narcoleptics (Vernet & Arnulf, 2009; Bassetti et al., 1997). Associated symptoms are headaches; orthostatic hypotension with syncope and peripheral vascular complaints

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(Raynaud´s type phenomena with cold hands and feet) but normally they do not require medical care (Bassetti et al., 1997). A study shows evidence that IH, particularly with long sleep time, is associated with evening type (chronotype) and young age (Vernet et al., 2009).

2.3.4. Etiology

The etiology is unknown. In contrast to narcolepsy, the disorder is not related to certain HLA types or Hcrt levels and a precipitating factor has not been identified. Guilleminault (2001) suggests the possibility that IH is a consequence of undiagnosed encephalitis but this is difficult to investigate.

2.3.5. Pathophysiology

Little is known about the pathophysiology of IH with and without long sleep time. A Norwegian study affirmed that narcolepsy patients without cataplexy could not be distinguished of IH by biochemical findings (Heier et al., 2007). There is evidence that IH patients have normal Hcrt-1 concentrations in the cerebrospinal fluid and a decreased histaminic transmission (American Academy of Sleep Medicine, 2005). A Czech study found a lower nocturnal level of melatonin in IH compared with controls (Nevsimalova et al., 2000). The same group found a phase delay in the rhythm of melatonin and cortisol secretion in patients with polysymptomatic form of IH (Nevsimalova et al., 2009).

Kanyabashi and collaborators found decreased histamine in the cerebrospinal fluid in 14 IH patients (Kanbayashi et al., 2009).

Regarding polysomnography data in IH patients, a research group hypothesized that there is a decreased slow wave sleep during the first two sleep cycles, suggesting that non restorative night sleep might be the cause of EDS (Sforza, Gaudreau, Petit & Montplaisir, 2000). However, this finding requires replication. Studies exist that have documented a consistent familial predisposition to IH with long sleep time (Billiard et al., 2001a).

2.3.6. Epidemiology

IH is a rare condition whose prevalence is unknown but is estimated to be about 10 times less than narcolepsy. There is no sex predominance (American Academy of Sleep Medicine, 2005) but one study reports a female: male ratio of 1.8:1 (Bassetti et al., 1997).

The beginning of the illness is in the first two decades of life (Anderson et al., 2007; Haba-Rubio, 2005; Kryger et al., 2005; Bassetti et al., 1997; Bruck et al., 1996).

2.3.7. Diagnosis

For the diagnosis of IH, the minimal criteria are a complaint of EDS, difficulty waking up in the morning or after a nap, duration at least six months, exclusion of other conditions that can explain the cause of the symptoms and the absence of a recent (within 18 months) head trauma. For the exclusion of conditions that may cause the symptoms of IH, it is necessary to perform sleep studies such as PSG and the patient must complete sleep diaries. The manual of sleep disorders advises for the correct interpretation of PSG that the recordings should be performed when the patient is free of drugs that influence sleep for at least 15 days. Seven days before, the sleep wake schedule must be standardized and nocturnal PSG must be performed on the night immediately preceding the MSLT (American Academy of Sleep Medicine, 2005).

An alternative method to diagnose IH is a continuous 24-h polysomnography where the subject is allowed to sleep ad libitum (Latin word that means “at will”). However, this method is until now not standardized with a large number of patients and normal controls to become reliable (Bastuji & Garcia-Larrea, 1999). Another method to assess sleep drunkenness is using evoked potentials but this technique has some inconveniences due to the inter-subjects variability (Bastuji et al., 1999).

DIFFERENTIAL DIAGNOSIS

IH is not easy to diagnose clinically since a variety of other causes of hypersomnia must be excluded. For instance, mood disorders, chronic fatigue syndrome, post-viral infections, endocrine disorders and other forms of excessive daytime sleepiness which are due to perturbed nocturnal sleep (Nevsimalova et al., 2000). Patients with the disorder may need a monitoring of esophageal pressure during sleep to exclude upper airway resistance syndrome and must have a psychiatric evaluation in order to exclude another reason for daytime sleepiness such as a medical or mental disorder that could explain the symptoms (ICSD-10). The presence of multiple brief arousals occurring periodically during PSG may suggest a need for monitoring esophageal pressure (American Academy of Sleep Medicine, 2005).

Mood disorders are part of the differential diagnosis of IH. This criterion is part of a dilemma because if hypersomnia symptoms come before the beginning of mood changes, it is not diagnosed (Billiard, 2001). Because of the similarity of the symptoms in some cases IH can be confused with “atypical depression” (Haba-Rubio, 2005). Therefore, it should be checked whether the onset of EDS is associated with a depressive or hypomanic state (Roth, 1980). Sleep drunkenness is uncommonly related with narcolepsy

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without cataplexy and can be a symptom also in subjects under sleep deprivation diagnosed with a phase delay syndrome (Nevsimalova et al., 2000).